In an image forming apparatus such as a laser printer, a plurality of printing modes are set in order to obtain the optimum image for dealing with a printing paper of a large number of types. It is so arranged that each printing mode is set by the user him/herself at the time of printing. This means that the user is required to have the knowledge for distinguishing the type of paper and that the user must set the apparatus to the type of paper him/herself. Another problem is that the best image will not be obtained if the setting is made erroneously.
An image forming apparatus that has become available in recent years detects the difference between the amount of regular reflected light, which is reflected by a paper surface, and the amount of diffused reflected light and discriminates the type of paper automatically, thereby obtaining the optimum image by performing control of image formation that conforms to the result of detection (e.g., see Patent Reference 1: Japanese Patent Application Laid-Open No. 11-216938).
FIG. 9 is a sectional view illustrating a printer glossmeter described in Patent Reference 1. As shown in FIG. 9, a gloss detector 200 has a block 210 mounted on a printed circuit board 220 in the usual manner. A light-source tube 212 on an axis 213 and a reflecting tube 214 on an axis 215 are formed inside the block 210. A light source 216 is situated inside the light-source tube 212 and a light sensor 222 is situated inside the light-source tube 212. The light sensor 222 reacts mainly to spectral reflected light and discriminates low-gloss paper and high-gloss paper.
Further, a technique for determining the coarseness of paper by capturing the surface image of the paper by a CCD area sensor and finding the fractal dimension has been proposed (e.g., see Patent Reference 2: Japanese Patent Application Laid-Open No. 11-271037).
FIG. 10 is a flowchart illustrating the basic operation of a smoothness detector described in Patent Reference 2. The surface of a printing medium is illuminated with light by area illumination (step S2-1). Next, a shadow image formed by the reflected light of area illumination is read as a planar image by image detecting means inclusive of image reading means, and grayscale information is detected as multivalued image data (step S2-2). More specifically, the reflected light resulting from the illuminating light is shadowed owing to unevenness of the printing medium, depressions appear dark and protrusions appear bright. The shadow image is detected by the CCD of the image reading means. The detected grayscale information, which is the multivalued image data, is subjected to image processing by information processing means, whereby surface roughness of the printing medium is measured and calculated (step S2-3). An image-formation parameter value corresponding to the surface roughness measured and calculated is thenceforth decided and controlled by image formation control means (step S2-4). That is, in this example of the prior art, the surface roughness of the printing medium can be inferred by reading the grayscale information from the CCD.
Furthermore, video of a paper surface is shot, information concerning the paper is acquired and image forming conditions are changed over (e.g., see Patent Reference 3: Japanese Patent Application Laid-Open No. 2002-182518).
By using the results obtained by discriminating types of printing media by these discrimination methods, an image forming apparatus performs printing upon selecting the printing modes that conform to the printing media of each type.
However, the surface conditions of a paper type differ depending upon the manufacturing lot and environment, and therefore the results of discrimination tend to vary. Further, even if many printing modes are provided in order to perform ideal image formation control in dependence upon various paper types, many types of printing paper are available on the market and therefore it is very difficult to set the correct paper-type mode without mistaking the paper type.
Accordingly, with a paper type in the vicinity of a threshold value, printing may be performed in the wrong mode if discrimination performed automatically is erroneous. If discrimination performed manually is erroneous, a case may arise in which printing cannot be performed.
Further, in a case where paper type has been discriminated using an image sensor such as an area sensor or line sensor, the amount of image data becomes very large and communication error becomes a possibility. If printing is halted whenever such anomalies occur, there is a possibility that the user will be subjected to stress.
Furthermore, with regard to printing on translucent media, there are cases where printing is performed without adding on a counterfeit preventing signal. Consequently, if plain paper is erroneously discriminated as translucent media, there is the danger that printing will be performed without printing the counterfeit preventing signal.